The present invention refers in general to vehicle headlights of the type comprising a plurality of light sources, for example LEDs, and in particular to an illumination arrangement for a vehicle headlight.
In the automobile sector, studies have been carried out for some time on new solutions for producing front and rear vehicle lights formed of matrices of LEDs (English acronym for “light-emitting diode”) or other light emitting devices, so as to obtain more compact devices, in particular in terms of depth, and with new aesthetic content.
As is known, conventional road lights are based on a lamp type of light source, of the halogen or discharge type, and an optical system capable of forming a light distribution according to the regulations in force. The optical system may consist of a complex reflector with segments and of a smooth transparent element, or of a substantially parabolic reflector with a prismatic transparent element, or of a combination of complex reflector and prismatic transparent element.
In each case, the reflector or the transparent element are respectively intended to reflect and transmit the light emitted by the light source. The transparent element defines the surface for the outlet of the light beam from the headlight.
As is likewise known, according to the existing regulations, a motor vehicle headlight must generate on a measuring screen disposed at a predetermined distance therefrom a specific light distribution.
This distribution, in the case of a dipped headlight, has a rectangular shape with reduced vertical divergence (typically less than 15°) and a markedly greater horizontal divergence (typically 60°–70°).
The dimensions of the headlight are particularly critical for the dipped function, inasmuch as the light distribution must exhibit a very abrupt transition from maximum illuminance to practically zero illuminance at the optical axis of the headlight.
This constraint means that the luminous flux is predominantly concentrated in a part of the distribution having a vertical angle of divergence typically below 5° and a horizontal angle of divergence typically below 20°–30°. FIG. 1 illustrates that part of the light distribution in which the flux for a dipped headlight is concentrated according to the European regulations (the distribution is tilted horizontally in the case of traffic on the left). This region is represented by a system of cartesian axes having their origin disposed on the geometric axis of the headlight. As can be seen, this region is horizontally centred with respect to the system of cartesian axes, has a horizontal angle of divergence of 20°, and extends vertically from the horizontal axis x substantially below the latter, with a vertical angle of divergence of 3°, exhibiting in addition a vertical lateral extension contained between the axis x and an inclined segment disposed above the latter. Typically, 40–50% of the luminous flux falls in the region shown in FIG. 1: for example, in a good quality vehicle headlight with discharge lamp, of around 700 lumens of flux emitted overall by the headlight, around 300 lumens finish in that region, while the remaining 400 lumens finish both in a region of the light distribution with an angulation of more than 3° and positioned below the region of distribution of FIG. 1, and in a region with an angulation of more than 20° and positioned laterally with respect to the distribution of FIG. 1.
The dimensions of the outlet surface of conventional dipped headlights are very variable, as is also the depth: the depth in particular is bound by the fact that the linear dimension of the source, for example of the filament in the case of an incandescent lamp, is typically never less than 4 mm; in order to guarantee the photometric performances disclosed above and provided for by the regulations, the reflector must have a focal distance typically of not less than 80 mm. The selection of a long focal distance in dipped headlights is substantially linked to the need to maintain the vertical angle of divergence of the light distribution within an interval of a few degrees; in fact, with L the vertical semi-dimension of the source, and θ the maximum angle of semi-divergence admissible in the vertical direction, the focal distance F of the system is defined by the equation F=L/tan(θ). The focal length F also restricts the dimension of the outlet cross-section of the headlight, inasmuch as the reflector must receive as much as possible of the flux emitted by the source.
A particularly compact version (in terms of outlet surface area) of a dipped headlight is the so-called “elliptical” headlight, consisting of an elliptical reflector which forms an image of the source; the abrupt cut-off in the distribution is in this case obtained by obscuring a part of the light emerging from the reflector through a diaphragm placed in proximity to the image of the source formed by the reflector. A final lens projects the image of the diaphragm in the far field forming the distribution provided for by the regulations. The presence of the diaphragm makes it possible to use reflectors with a smaller focal distance (and therefore forming images of the source of greater dimensions); the price paid is a greater depth of the reflector and a lesser efficiency (typically below 35%, as against the 70% and more of a conventional headlight of the non-elliptical type).
In order to obtain headlights with a lesser focal distance and therefore a lesser thickness, the only solution is to reduce the vertical dimension of the source, which can be done either by using sources with greater emittance (i.e. flux emitted by the unit of surface area), or by dividing the source into a multiplicity of sources of smaller vertical dimension, in such a manner that, with parity of emittance, the sum of the areas of the individual sources is equal to the surface area of the original source, thus obtaining the same overall flux.
A typical source for motor vehicle headlights is the halogen lamp, with a power of 55 W (for example the category H7 lamp), with a nominal flux greater than 1300 lumens and an equivalent emitting surface area greater than 20 mm2, which is equivalent to an emittance of 65 lm/mm2.
White LEDs currently have a maximum emittance of 18 lm/mm2, although LEDs of up to 25 lm/mm2 are beginning to be available commercially. It is thought that the white LED may in a few years reach the threshold of 40 lm/mm2, owing to the rapid progress in the field of technology and of semiconductor junctions and of phosphors, as well as to the continuous improvements in packaging technology. The aim of the LED constructors is that of reaching, in the medium-long term, emittances of up to 100 lm/mm2.
In the case of incandescent micro-sources of reduced dimensions, the solution of increasing the emittance cannot easily be pursued, inasmuch as the emittance of an incandescent source is bound by the operating temperature through the Planck law. Increasing the emittance means in practice raising the temperature of incandescence, which nowadays is already at limit values, compatibly with an acceptable average service life; however, up to now, halogen lamps still have an emittance 3–4 times higher than the white LED.